nouveau: add an ioctl to return vram bar size.

This returns the BAR resources size so userspace can make
decisions based on rebar support.

userspace using this has been proposed for nvk, but
it's a rather trivial uapi addition.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>